The so-called “lost-foam” casting process is a well known technique for producing metal castings wherein a fugitive, pyrolizable, polymeric, foam pattern (including gates, runners, risers, etc.) is covered with a thin (i.e. 0.25-0.5 mm) gas-permeable, ceramic coating, and embedded in an unbonded sand mold to form a mold cavity within the sand. Molten metal (e.g. iron or aluminum inter alia) is then introduced into the mold, from above or below, to pyrolize, and displace the pattern with molten metal. Gaseous and liquid pyrolysis products escape through the gas-permeable, ceramic coating into the interstices between the unbonded sand particles. Typical fugitive polymeric foam patterns comprise expanded polystyrene foam (EPS), polymethylmethacrylate (PMMA), and certain copolymers.
A new pattern is needed for each casting, and must accurately duplicate the dimensions and shape of the casting to be produced. The dimensional accuracy of the castings is no better than the dimensional accuracy of the patterns that produce them. If there are variations between the patterns, or if there are differences between the actual pattern dimensions and the intended pattern dimensions, it will be necessary either to scrap the patterns if they are too small, or to perform extra machining of the castings if they are too large.
While the invention is described hereinafter in terms of patterns made from EPS beads having a pentane (i.e. n-pentane, isopentane and/or cyclopentane), blowing agent, it is to be understood that the invention is likewise applicable to other polymeric foams and blowing agents. To make EPS lost-foam patterns, partially pre-expanded EPS beads are blown into a mold, and therein subjected to steam to complete their expansion and fuse them together into a unitary mass. More specifically, raw so-called T-size (i.e. 0.25 mm diameter, 40 lbs./ft3 density) EPS beads are formed from polystyrene containing about 6.0 percent by weight of a pentane blowing agent. The raw beads are too dense, and contain too much blowing agent, for “as is” use in a one-step pattern-molding operation. Accordingly before molding the patterns, it is common practice to subject the beads to a pre-expansion operation wherein they are heated, and partially expanded to reduce their density to about 1.4 lbs/ft3, and their pentane content to about 4%-5%. Pre-expansion equipment is commercially available for this purpose. In one pre-expansion technique, the raw beads are charged into a closed vessel having an expansion chamber where they are contacted with saturated steam at low super atmospheric pressure, which gasifies the pentane, expands the beads diametrically fourfold (i.e. to about 1 mm), and drives off some of the pentane. Thereafter, the beads are cooled by standing, or by fluidization in ambient air. The pre-expanded beads have a cellular structure, and are close to the bead size used to mold the patterns. In another pre-expansion technique, the beads are not directly exposed to the steam, but rather are heated at about 200° F. in the expansion chamber through contact with a steam jacket, to expand and partially degas (i.e. remove pentane) them, and then water-cooled to arrest their expansion. The pre-expanded beads are then screened to remove any unexpanded raw beads and/or any clumps of beads that might have formed.
The mold used to make the pattern from the pre-expanded beads has perforate walls defining a mold cavity through which 240° F. steam enters the mold cavity. In the “flow-through” steaming technique, steam is introduced, through one of the mold segments on one side of the mold cavity, passes through the beads in the mold cavity, and exits the mold cavity through the other mold segment on the opposite side of the mold cavity. An alternative steaming technique is known as “autoclaving” which involves pressurizing both segments at the same time so as to soak the beads in the steam for a sufficient duration to expand and fuse them together. Following steaming, the pattern is cooled to about 140° F. by spraying water onto the backside of the mold segments, and/or by the application of vacuum to the steam chests until the expansion of the beads is arrested.
In an attempt to provide reproducible, dimensional stability from one pattern to the next, and thus from one casting to the next, it was an early practice to age the pre-expanded beads before molding to obtain a prescribed pentane content in the beads, and to age the patterns after molding so that they all come to a reproducible final configuration/dimension. In one practice, the pre-expanded beads were stored for 24 hours before pattern molding, and the patterns were stored from 6-72 hours before use to allow their pentane content to stabilize, and any water therein to evaporate. Shorter aging times (e.g. 2-6 hours) were made possible by force-aging the patterns in a circulating-air oven at a temperature of about 165° F.
To avoid significant variations in the pentane content of the pre-expanded beads being supplied to the pattern-molding machine, Bishop U.S. Pat. No. 5,385,698 (which is assigned to the Assignee of this invention, and is hereby incorporated by reference) proposed a technique for making dimensionally accurate lost foam patterns without the need for extended aging of either the pre-expanded beads or the finished patterns. Bishop seeks to control the pentane content of the pre-expanded beads supplied to the pattern-molding machines within defined limits, by conditioning them for about 60 to 90 minutes in a so-called “pentane reduction chamber” (hereafter PRC)”. In the PRC, a stream of hot (preferably ca. 170° F.) air fluidizes the beads, and strips away any excess pentane therefrom so as to provide pre-expanded beads that have a pentane content less than 3.75%. While the invention is described herein in the context of “air” as being the fluidizing gas in the PRC, it is to be understood that other gases, such as nitrogen, helium, argon, CO2, etc., that are unreactive with styrene or pentane, and that do not interfere with the quantitative, spectroscopic analysis of pentane in the presence of polystyrene, may be substituted for some, or all, of the air.
It has been shown that the degree of pattern fusion influences the way the metal fills the casting cavity in the sand. Poor fusion, resulting from too low pentane content, results in turbulent metal fronts that move erratically through the foam pattern causing porosity and poor casting quality. Good fusion, resulting from higher pentane content, causes a more controlled metal front and fewer casting defects. By controlling the amount of pentane in the beads to provide consistent levels of pentane in the beads supplied to the pattern mold, more consistent levels of fusion are obtained, and consequently more consistent casting quality.
Bishop attempts to control the pentane content of the pre-expanded beads by fixing the duration of the hot-air-fluidization step undertaken in the PRC. The duration chosen is based on day-to-day practical experience operating the PRC, and empirical, off-line (i.e. tests conducted remote from the bead handling/conditioning equipment) thermo-gravimetric tests wherein samples of the beads are weighed, heated to drive off the pentane, and weighed again to determine their weight loss. Alternative, off-line, analytical tests such as gas chromatography or NIR-spectroscopy have been proposed by others.
Tighter control of the pentane content of the pre-expanded beads than is possible with the Bishop technique is needed to keep the pentane content within a preferred prescribed narrow range of about 1.4% to 2.5% by weight. The present invention is directed toward making patterns for the Lost Foam process that are dimensionally stable/consistent by the periodic, real-time, online (i.e. pentane analyzer is coupled directly with the bead handling/conditioning equipment for closed-loop control of pentane content), measurement of the pentane content in the pre-expanded beads used to make the pattern, and in response thereto, automatically either (a) adjusting the operating conditions of the PRC to keep the pentane content within the prescribed range, (b) adjusting the pattern-molding conditions to accommodate beads whose pentane-content is outside the prescribed range, or (c) shutting down the pattern-mold(s) when the pentane content of the beads enroute to the pattern mold falls outside the prescribed range. Real-time (i.e. within about 15 secs.) on-line, measurements of the pentane content of the beads may be taken in situ at various sites throughout the bead handling/conditioning apparatus/equipment (e.g. in the PRC, storage vessels or bead transfer ducts/lines), using a near-infrared (NIR) spectroscopic sensor coupled with the equipment at such sites.